Enable addition of custom audio outputs

My local MPD configuration:
```
	audio_outputs		=>
	 	[
			# Audio outputs docs:
			# http://mpd.wikia.com/wiki/Configuration#Audio_Outputs
			{
				'name' => 'Local PulseAudio',
				'type' => 'pulse',
			},
			{
				# Workaround for problem in MPD output to local PA:
				# https://wiki.archlinux.org/index.php/Music_Player_Daemon/Tips_and_Tricks#Local_.28with_separate_mpd_user.29
				'name' => 'Local PulseAudio (TCP)',
				'type' => 'pulse',
				'server' => '127.0.0.1',
			},
			{
				'name' => 'PulseAudio in office',
				'type' => 'pulse',
				'server' => 'office',
			},
			{
				'type' => 'httpd',
				'name' => 'HTTP Stream (port 8000)',
				#'encoder' => 'vorbis',		# optional, vorbis or lame
				'port' => '8000',
				#'quality' => '5.0',		# do not define if bitrate is defined
				'bitrate' => '128',			# do not define if quality is defined
				'format' => '44100:16:1',
			},
		],
```
and it's working great :)